Section 2. Such dam shall not be erected to the injury of a mill lawfully existing above or below it on the same stream, nor to the injury of a mill site on the same stream on which a mill or mill dam has been lawfully erected and used, unless the right to maintain a mill on such last mentioned site has been lost by abandonment or otherwise; nor shall a mill dam be erected or raised to the injury of any such mill site which has been occupied as such by the owner thereof, if, within a reasonable time after commencing such occupation, he completes and puts in operation a mill for the working of which the water of such stream is applied; nor shall a mill or dam be placed on the land of any person without such grant, conveyance or authority from the owner as would be necessary to convey land.